Heretofore, it has been known to utilize a rotary kiln to heat treat materials. Such a kiln can be used to reduce materials such as the oxides or sulfides of metallic materials, such as nickel oxide, to a metallic powder by heating the material at a high temperature in a cylindrical rotary kiln in the presence of a reducing agent where the ore is at least partially reduced. Upon leaving the rotary kiln, the ore passes to an electric furnace where it is further reduced to the condition of a molten alloy, the impurities being removed in the form of slag. The prior art rotary kiln utilized in such a process has a first end and a second end, with the first end being for receipt of the materials and the second end for the discharge of the reduced materials. The kiln is inclined generally downwardly and is rotated to advance the materials through the interior of the kiln from the first end to the second end so that, upon rotation, the materials to be reduced will tumble and mix as they advance through the kiln. A reducing medium, such as natural gas substantially free of any oxidizing gases, is primarily introduced into the interior of the kiln through ports near the first end and moves in countercurrent fashion to the direction of movement of the material in the kiln. The gas passes through the material to make initial and intimate contact with material. The contact of such gases with the materials in an oxide or sulfide form at such a high temperature causes a portion of the gas to quickly pyrolytically decompose with an amount thereof being reformed as a strong reducing agent including, for example, carbon monoxide and hydrogen.
The burner in the rotary kiln used to generate the necessary high temperatures will typically project from the discharge end of the kiln axially into the kiln and is fed with liquid, solid or gaseous fuel. A significant issue with conventional reduction kiln technology is the need to also provide combustion air, e.g. oxygen or oxygen-air mixtures at ambient temperature at the kiln product discharge. This practice results in an oxygen rich, cold air stream passing directly over the heated, reduced kiln material prior to discharge. This cold, oxygen rich air cools the product significantly and also oxidizes a portion of the product, thus negatively affecting the amount of reduced material realized via the operation. This, in turn, increases the work necessary in subsequent processing steps, in particular within the downstream electric furnaces.
It is therefore an object of the present invention to provide an improved method of reducing a material in a rotary kiln to its corresponding metal and delivering a higher temperature product to downstream processes such as electric furnaces.
It is a further object of the invention to provide a rotary kiln which, particularly in reducing metallic oxides or sulfides, will facilitate the production of the corresponding metal at a greater yield than realized by the prior art processes described above.